With indirect heating the lack of means of regulating the steam-temperature is not of so much moment, for the air-supply can be partly shut off by partly closing a register in mild weather; or else, if the full air-supply is re quired at all times, arrangements can be made for passing part of the air-supply around the indirect radiators, which is called aby-passine them. Another method is to divide the indirect radiator into independent sections and place some of the sections under the control of a regulator that automatically shuts off the sup ply of steam when the room becomes too warm. The method of °by-passing° the radiator, or subdividing it, is used mainly with the fan system of supplying air.
The cost of indirect hot-water heating is greater than that of indirect heating by steam, as the radiators and pipes must be larger, the same as in direct heating. Hot water is, how ever, cheaper to operate. The principal objec tion to its use in indirect heating is the possi bility of damage to the indirect radiators through the freezing of the water in them in severe weather, if the circulation should from any cause be arrested.
The direct-indirect system consists of direct radiators connected with the outer air by means of an opening in the building-walls beneath the window-sill, the radiator being set under the window opposite the opening. With this sys tem there is always the possibility of getting too much air when the wind blows strongly. Fur thermore, in situations where the air is smoke laden or dusty, it is not easy to keep the smoke and dust from entering a building supplied with air by this means.
As has been said, a supply of air may be brought about by the gravity-method or by means of fans. In the gravity-method the heated column of air in the flue is lighter than the outdoor air; hence it rises. As in the case of hot-water heating, the motive power is very slight, and it becomes less as the outdoor tem perature increases. For this reason the gravity system is not a positive one, and it cannot be depended upon to supply much air in mild weather. Its use for schoolhouse ventilation is therefore to be deprecated. An important advantage of this system is its simplicity, as no machinery is required with it.
With the fan-system some type of fan is em ployed, to give a positive supply of air. The air is blown over coils, usually steam, and de livered to the room at a temperature slightly above that of the room, if the air-supply is in tended to ventilate only, or at a higher temper ature if the air-supply is to carry with it the heat necessary to balance that transmitted by the walls and windows. In the former event
the indirect coils act as tempering-coils, being sufficient only to raise the air to about 70° F. If the air-supply is to furnish heat for warming the rooms, additional coils, known as supple mentary coils, are provided. These raise the air-temperature from 70° to from 100° to F. Sometimes the supplementary coils are com bined with the tempering coils, the whole being divided into several independently controlled sections. In some instances the supplementary coils are divided into a number of small coils, one being placed at the base of each air-supply flue, and so arranged that, by adjusting dampers controlled by hand or automatically, the tem perature of the air supplied to any room can be regulated independently of that supplied to other rooms. If all of the air is passed through one group of coils, independent regulation of the temperature of the air in the branch ducts and flues is impossible. This independent regula tion can be obtained, however, by the double duct system. The coils are divided into two groups, one for tempering and one for supplying additional heat. All of the air is passed through the tempering coils, but only part of it through the supplementary coils, the balance Thy-passing* the latter coils and flowing through a system of ducts, usually located below the system con veying the air of higher temperature, to the base of the flues. At the junction of the two ducts a mixing-damper is provided, so arranged as to open in one duct as it closes in the other. By adjusting this damper the air can be mixed to give the resultant temperature required.
In situations where direct radiators can be used, either exposed or concealed, it is becom ing the practice to provide sufficient heat by means of direct radiation to balance the heat transmitted by walls, windows, etc., also a sup ply of tempered air for ventilation only. As previously explained, when heat is supplied by means of air, the fuel-cost is greater than with direct heating; so that a building can be warmed with less coal with the direct than with the indirect system. Furthermore, with the com bined system, heating can be done at night, and at other times when air-supply is not required, at minimum cost. This system is particularly adapted for schoolhouse heating and ventilation.